Polyolefin foams are produced by modifying a raw polymer material by introducing a dispersed-gas phase into the structure of the polymer. This process is conventionally performed by dispersing a blowing agent and cross-linking agents into a polymer melt and exposing the resulting compound to an environment where the blowing agent decomposes thereby releasing the gas and expanding the polymer.
Cross linking modifies the physical characteristics of the expanded foam product, the primary characteristics being the cell size and degree of cell closure.
To the present time, methods of producing polyolefin foam include forming calendered or otherwise preformed flat sheets which are then expanded in a one or two stage process. The sheets are heated in a mould to activate a decomposable, gas-releasing blowing agent. This process will cause the calendered or otherwise formed sheets to expand to up to approximately twenty times their original volume.
The heating and subsequent blowing agent activation step may be carried out in two steps using two different sized moulds. The sheets may be blown in one mould to an intermediate size following which they are blown to their final size in a second mould. The intermediate moulding step helps limit the stresses produced in the foam during the expansion process. The sheets must be blown in restraining moulds as flat sheets distort during blowing by expanding more in the centre and along the long sides. This is due to variations in surface tension of the foam surface. Surface tension effects are less significant in these regions than at the corners.
Other techniques include single step extrusion of planks, rounds and special purpose profiles. In this process, the extrusion of the polymer-blowing agent mixture is conventional, however before the mixture is allowed to expand it is passed through, for example, an annular channel. As the mixture leaves the channel, it is expanded. The product can be expanded to useful volumes, however the extruded product incorporates integral nonexpanded skins which can reduce the overall density by about a half.
Further objects and advantages of the invention will become apparent from the following description which is given by way of example only.
The object of the present invention is to provide alternative methods and apparatus for producing moulded polyolefin foams, said methods and apparatus having reduced energy requirements compared to conventional processes and employing a simplified and compact mould construction requiring reduced clamping pressures or to at least provide the public with a useful choice.